Thermally developable photosensitive materials are known, and thermally developable photosensitive materials and the processes involved have been described, for example, on pages 242-255 of Fundamentals of Photographic Engineering, Non-silver Salt Photography Section (published by Corona, 1982) and in U.S. Pat. No. 4,500,626.
In addition, methods for the formation of dye images by means of a coupling reaction between the oxidized form of a developing agent and couplers have been described, for example, in U.S. Pat. Nos. 3,761,270 and 4,021,240. Furthermore, methods for the formation of positive color images using a photosensitive silver dye-bleach system have been described, for example, in U.S. Pat. No. 4,235,957.
Methods in which diffusible dyes are released or formed in the form of an image by thermal development and in which the diffusible dyes are transferred to a dye fixing element have been proposed more recently. With these methods, it is possible to obtain both negative dye images and positive dye images by changing the type of dye donating compounds which are used or by changing the type of silver halide which is used. More details have been disclosed, for example, in U.S. Pat. Nos. 4,500,626, 4,483,914, 4,503,137 and 4,559,290, JP-A-58-149046, JP-A-60-133449, JP-A-59-218443, JP-A-61-238056, European Patent laid open 220746A2, Kokai Giho 87-6199 and European Patent laid open 210660A2. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".)
Many methods have also been suggested for obtaining positive color images by thermal development. For example, a method has been proposed in U.S. Pat. No. 4,559,290, in which compounds for which so-called DDR compounds have been formed into a oxide form which has no dye releasing capacity and a reducing agent or precursor thereof are included, the reducing agent is oxidized in accordance with the exposure of the silver halide by thermal development, reduction is achieved with the remaining un-oxidized reducing agent and a diffusible dye is released. Furthermore, thermally developable photosensitive materials in which compounds which release diffusible dyes by a reductive cleavage of an N--X bond (where X represents an oxygen atom, a nitrogen atom or a sulfur atom) are used as compounds which release a diffusible dye via the same mechanism, have been disclosed in European Patent laid open 220746A and in Kokai Giho 87-6199 (Vol. 12 No. 22)
When photographically useful additives, such as dye donating compounds, which are insoluble in water are added to a photosensitive material such as those mentioned above, the addition is generally made by the method of emulsification and dispersion which is well known to those in this field. In the emulsification and dispersion method, the photographically useful additive is dissolved together with a high boiling point organic solvent in a low boiling point organic solvent, and the solution is added to an aqueous gelatin solution in the presence of an appropriate surfactant and emulsified and dispersed as an O/W type emulsion using an emulsifying machine such as a homogenizer for example, and such methods have been disclosed, for example, on pages 213-255 of Fundamentals of Photographic Engineering, Silver Salt Photography Section (published by Corona, 1978) and U.S. Pat. No. 2,322,027. Coating liquids which contain emulsions which have been prepared using the abovementioned method of emulsification and dispersion are not used immediately after preparation. Rather they are used after being kept warm and ageing after preparation. But when a coating liquid which has been kept warm and aged in this way is used, there are cases where large lumpy particles are formed by cohesion of the emulsion particles and by precipitation of the photographically useful additives which are insoluble in water such as the dye donating compounds for example, and spot-like defects are produced in the image and discrimination becomes poor.